The use of two or more extrusion screws in a common housing, generally of steel, is advantageous for the effective mastication and conveyance of considerable quantities of thermoplastic material. With thermally unstable plastics it is generally necessary to use deep-cut screws with interleaved threads counterrotating at rather slow speeds under a large torque.
The small spacing between the axes of such coacting screws creates problems in accommodating the necessary speed-reduction gearing by which they must be connected to a common power shaft for rotation in unison. Difficulties are particularly experienced with the mounting of the transmission output shafts directly connected with the respective screws coaxial therewith. The large torques to be transmitted to the extrusion screws via intermeshing gears generate considerable pressures at the gear teeth which tend to deform the output shafts driving the screws and lead to a loss of parallelism of these shafts in their highly stressed bearings. The latter, in turn, must be restricted in size because of the limited space available for their emplacement. It is also not possible to increase at will the axial width of the gears, in order to reduce the pressure per unit area, since this would unavoidably bring about a nonuniform contact between the tooth flanks of the driving and driven gears; this problem is solved only in part, and at the cost of an increased axial length, by the use of several juxtaposed pairs of gears with oppositely inclined teeth.